Changes in surface hydrology, soil moisture and gross primary production in the Amazon during the 2015/2016 El Niño

Abstract

The 2015/2016 El Niño event caused severe changes in precipitation across the tropics. This impacted surface hydrology, such as river run-off and soil moisture availability, thereby triggering reductions in gross primary production (GPP). Many biosphere models lack the detailed hydrological component required to accurately quantify anomalies in surface hydrology and GPP during droughts in tropical regions. Here, we take the novel approach of coupling the biosphere model SiBCASA with the advanced hydrological model PCR-GLOBWB to attempt such a quantification across the Amazon basin during the drought in 2015/2016. We calculate 30–40% reduced river discharge in the Amazon starting in October 2015, lagging behind the precipitation anomaly by approximately one month and in good agreement with river gauge observations. Soil moisture shows distinctly asymmetrical spatial anomalies with large reductions across the north-eastern part of the basin, which persisted into the following dry season. This added to drought stress in vegetation, already present owing to vapour pressure deficits at the leaf, resulting in a loss of GPP of 0.95 (0.69 to 1.20) PgC between October 2015 and March 2016 compared with the 2007–2014 average. Only 11% (10–12%) of the reduction in GPP was found in the (wetter) north-western part of the basin, whereas the north-eastern and southern regions were affected more strongly, with 56% (54–56%) and 33% (31–33%) of the total, respectively. Uncertainty on this anomaly mostly reflects the unknown rooting depths of vegetation.This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications’.

Towards a global understanding of vegetation–climate dynamics at multiple time scales

“…Regions standing out through heterogeneous patterns, such as the Amazon, should be further investigated at temporally and spatially higher resolution to better understand local influence of climate, vegetation and topography on atmosphere-biosphere co-variation. Recently, studies in the Amazon based on products such as SIF have detected differences in vegetation anomalies within the basin during El Niño events (Koren et al, 2018). The identified asymmetry in the East-West gradient coincides with observed changes in temperature, soil moisture and GRACEderived water storage.…”

“…The identified asymmetry in the East-West gradient coincides with observed changes in temperature, soil moisture and GRACEderived water storage. Our results pave a way for better understanding the spatial heterogeneity of ecosystem responses to climate variability (van Schaik et al, 2018). Here, assessing temporal patterns beyond correlation (c. f. Wu et al, 2015) will provide additional insights into the temporal evolution of vegetation dynamics, and the carbon cycle variability.…”

“…We find heterogeneous temporal patterns of biosphere-climate responses across time scales. The emerging patterns should be compared with newer satellite products such as sun-induced fluorescence (SIF), which are coupled more directly to plant physiology and photosynthesis (Badgley et al, 2017;Koren et al, 2018), but are only available for short time periods. Considering further variables influencing vegetation dynamics, such as radiation, cloud cover, soil moisture, fires, or storms could bring additional insight into the drivers of vegetation dynamics, especially for poorly explained regions in the current analysis, such as the tropics.…”

Feedback between drought and deforestation in the Amazon

“…We then replaced these forest evapotranspiration fluxes by those for rainfed cropland (such as soybean plantations) [34], maximized at the original fluxes. The output of PCR-GLOBWB shows good agreement with independent dry-season evapotranspiration estimates [29] and discharge data [41] from the Amazon river. We show the differences in evapotranspiration between forest and rainfed cropland on a monthly basis in the supplementary material (figure S6).…”

Sources of Uncertainty in Regional and Global Terrestrial CO2 Exchange Estimates

“…The combination of these two results hints at differences in the sensitivity of NBP from inversions and DGVMs to changes in water availability or temperature related with ENSO (section ), or possibly also to changes in atmospheric circulation/mixing patterns linked to ENSO. The opposing sign in the sensitivity to temperature between inversions and DGVMs (Figure ) may be due to an overestimate of the sensitivity of respiration to temperature (Bastos et al, ; van Schaik et al, ), or indirectly linked to the higher sensitivity of NBP to water availability in DGVMs compared to inversions. Models differ in prescribed soil depth and root water access for transpiration, but in general do not have deep rooting and ground water access of plants (Fan et al, ), which may explain why the sensitivity of regional CO fluxes to water availability in DGVMs is higher than that of inversions with TWS.…”